Thalassemic red blood cells (RBCs) are known to have shorter survivals with accompanying numbers of metabolic defects and virtually all kinds of morphologic abnormalities are described. They are under continuous oxidative stress with exhausted antioxidative components. It is anticipated that the excessive uses or insufficient supplies of high energy phosphate (ATP) in thalassemic RBCs may be a presumptive factor that normally requires for maintaining the well being of RBCs. This study was designed to determine the metabolic status of thalassemic RBCs and assess in vitro if the effect of inosine, a known metabolic activator shows any phar-macologic benefit. Heparinized blood of normal volunteers, non-splenectomized and splenectomized thalassemias (THAL) were subjected to various conditions in vitro. Levels of ATP and FDP (furctose 1, 6-diphosphate, an intracellular glycolytic regulator) were respectively determined by HPLC and enzymatic methods. It was found that the levels of ATP in thalassemic RBCs of both non-splenectomized (NS) and splenectomized (SP) were lower than those of volunteers (NORMAL). The elevated FDP levels in the thalasssemic RBCs suggested that they were under continuous stress of having to compensate for the ATP-depleted RBCs. Inosine in the dose of 10 mM was found effectively in elevating both ATP and FDP in the RBCs, especially in ATP-depleted RBCs, of NORMAL and THAL. However, the restoring ability of inosine on the levels of intraerythrocytic ATP in THAL was not as marked as that in the NORMAL though the levels of FDP were higher elevated in THAL than that of NORMAL. These findings demonstrate that inosine is effective in maintaining the metabolic status of human RBCs of both NORMAL and THAL in vitro, and suggest that specific biochemical defect(s) may have occurred to the glycolytic regulatory path-way in the thalassemic RBCs.